Scumming

/ˈskəmɪŋ/

General Definition

In offset lithography, scumming refers to a condition where non-image areas on the printing plate begin to attract ink. Consequently, unwanted spots or patches of ink are transferred onto the paper surface. This issue is typically caused by a failure of the fountain solution to function correctly, meaning the area intended to remain hydrophilic (water-loving) loses its ink-repelling properties.

Real-World Usage

Understanding the concept of scumming is critical for print operators as it allows them to diagnose the precise root cause of unwanted marks on the final product. This phenomenon can manifest in various forms, starting as small, isolated spots, or quickly spreading across larger, non-image areas. If left unchecked, this widespread contamination can result in the complete spoilage of numerous print sheets and substantial material waste.

The ability to identify scumming early enables rapid corrective action to the fountain solution's settings, whether it involves adjusting the pH, conductivity, or water balance. Furthermore, the term 'scumming' is a necessary piece of professional jargon, frequently used in technical press reports, shift logs, and when consulting with technical representatives from ink, chemical, or plate suppliers.

Consultant's Note

If you detect scumming during the press run, your immediate action must be a comprehensive review of the fountain solution's chemical and physical properties:

First, the operator must verify the solution's acidity level. This is done by measuring the pH using a specialized pH Meter or pH test strips. The ideal reading must be kept within a tight, slightly acidic range (typically between 4.8 and 5.2) to maintain the plate’s integrity. Secondly, ensure the solution temperature is stable; the chiller must maintain the solution in the lower temperature range (usually 8 to 12 degrees Celsius). If the temperature is too high, volatile components like Isopropyl Alcohol (IPA) or its substitutes evaporate rapidly, destabilizing the solution's delicate balance and immediately causing scumming.

Finally, while addressing the core chemical imbalance, the operator may perform temporary mechanical adjustments. This involves slightly increasing the dampening level (water flow) via the machine's control console. This temporary boost of water helps to reinforce the plate's ink-repelling quality and mitigate the immediate issue until the chemical parameters are fully stabilized.

Aligned with: ISO 12647-2 (Offset) / Press Chemistry Standards / FOGRA